1. Field of the Invention
The present invention relates generally to a handover method in a mobile communication system, and in particular, to a handover method for a mobile station having a mobile IP address.
2. Description of the Related Art
Handover is implemented to ensure mobility for a mobile station and continue communications without interruption. The handover maintains a call and changes a source base station to a target base station without being noticed by a user. The mobile station continues the call, performing a control operation as the target base station becomes its serving base station. The handover is categorized into a soft handover and a hard handover. The Soft handover occurs with simultaneous channel assignment to two base stations, whereas the hard handover connects a channel to a target base station, releasing an established channel from a source base station.
Research has actively been made on assignment of a mobile IP (Internet Protocol) address to a mobile station to reliably provide an Internet service, a voice service and other data services, while ensuring mobility for the mobile station. With a mobile IP address assigned to a mobile station, the mobile station can receive services in other countries, such as the U.S., although the mobile station is registered in a home country, such as Korea. The mobile communication system is provided with a home agent (HA) to locate the mobile station, and a base station or a base station controller in another area acts as a foreign agent (FA). When the mobile station is located in another area, it registers its location in the FA and transmits the address of the HA as well as its registration and other information to the FA. The HA stores the location information of the mobile station, that is, the address of the FA, to thereby perform location registration for the mobile station.
The mobile station can then communicate through the FA. A corresponding node (CN) with which the mobile station communicates, whether it is a mobile station or a system (host), transmits/receives data over an IP network. To transmit data to the mobile station, the CN transmits the data to HA through a predetermined router, such as, an edge router connected to the IP network. The router then transmits data received from CN for the mobile station to the HA. The HA sends the data to the FA using the address stored during the location registration. Thereafter, the router becomes a node in a transmission path in which the CN transmits data to the mobile station through the HA. Meanwhile, since the FA can detect the address of the edge router, the CN can receive data directly from the FA. Therefore, a reception path from the FA to the CN is directly connected to the edge router. In this case, the transmission path is different from the reception path.
Alternatively, if mobile station wants to transmit some data to a CN, the mobile station transmits data simply to CN through FA by IP packet routing in the routers. When the mobile station receives data from the CN, the data will be also transmitted by IP packet routing in the routers.
Even when a data tranmission path is connected between the mobile station and a particular terminal in this manner, data is exchanged between the router connected to the CN and the FA of the mobile station by IP tunneling.
The mobile station may move to another FA without staying in one FA because it has mobility. In some cases, a reliable handover by IP tunneling cannot be implemented for the mobile station. More specifically, if the mobile station moves to a second FA with a data transmission path connected to a first FA, its location registration must be performed again. Then, the first FA hands over the data to the second FA. Here, the first FA is in the state where it transmits/receives data to/from the router connected to the CN by IP tunneling. However, there is no way for the first FA to notify the second FA that data can be exchanged between the CN and the second FA by IP tunneling, thereby impeding active handover implementation.